VAV Fumehood

VAV fumehood is the newest generations of laboratory fumehoods, vary the volume of room air exhausted while maintaining the face velocity at a set level.

Different VAV hoods change the exhaust volume using different methods, such as a damper or valve in the exhaust duct that opens and closes based on sash position, or a blower that changes speed to meet air-volume demands.

Most VAV hoods integrate a modified bypass-block system that ensures adequate airflow at all sash positions. VAV hoods are connected electronically to the laboratory building’s HVAC, so hood exhaust and room supply are balanced. In addition, VAV hoods feature monitors and/or alarms that warn the operator of unsafe hood-airflow conditions.

Although VAV hoods are much more complex than traditional constant-volume hoods, and correspondingly have higher initial costs, they can provide considerable energy savings by reducing the total volume of conditioned air exhausted from the laboratory. Since most hoods are operated the entire time a laboratory is open, this can quickly add up to significant cost savings.

This savings are, however, completely contingent on user behavior: the less the hoods are open (both in terms of height and in terms of time), the greater the energy savings Potential behavioral savings from VAV fume hoods are highest when fume hood density (number of fume hoods per square foot of lab space) is high.

This is because fume hoods contribute to the achievement of lab spaces' required air exchange rates. Put another way, savings from closing fume hoods can only be achieved when fume hood exhaust rates are greater than the air exchange rate needed to achieve the required ventilation rate in the lab room.

For example, if you have a lab room with a required air exchange rate of 2000 cubic feet per minute (CFM), and that room has just one fume hood, which vents air at a rate of 1000 square feet per minute, closing the sash on the fume hood will simply cause the lab room's air handler to increase from 1000 CFM to 2000 CFM, thus resulting in no net reduction in air exhaust rates, and thus no net reduction in energy consumption

With a vav fumehood the design and construction are similar to those for bypass fumehoods. This fumehood will maintain a constant face velocity by adjusting the the total exhaust airflow as the sash opens or closes.

They rely on more sophisticated control hardware and algorithms for laboratory HVAC and exhaust systems, for fumehood exhausts and fumehood controllers, and ultimately attempt to minimize energy costs while still protecting the fumehood user. Some bypass fumehoods can be retrofitted for use as VAV hoods by the use of a blank off plate or restricted bypass opening, which effectively eliminates or reduces the bypass air entering the fumehood.

The hood may be served by an individual fan which is speed controlled or a manifold duct system with VAV control device. The sash opening information is usually sensed by the position of a "string" positioning a potentiometer therefore providing feedback to the controls.

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